1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and in particular to a method for fabricating a mixed-signal semiconductor device.
2. Description of the Background Art
A mixed-signal semiconductor device represents a semiconductor device in which a digital signal and an analog signal co-exist. As a digital signal generation unit, a transistor is known, which generates an ON and OFF signal based on whether a threshold voltage is exceeded or not. As an analog signal generation unit, a capacitor is known, which generates a signal classified based on the stored amount of electric charges. Since the capacitor stores an electric charge between two polysilicon films which are called as a plate poly and a gate poly, the structure of the same is called as a double poly structure. This structure is generally used for designing an analog circuit. The method for fabricating a mixed signal semiconductor device formed of the transistors and capacitors using the above-described double poly structure is well known. The above-described conventional fabrication method will be explained with reference to FIGS. 1A through 1E.
As shown in FIG. 1A, a buffer oxide film 2 is formed on an upper surface of a semiconductor substrate 1, and a trench 3 is formed on the semiconductor substrate 1 based on a STI(Shallow Trench Isolation) process, and an oxide 4 is filled in the trench 3. The region of the semiconductor substrate 1 is divided into an active region A and a field region F, and a well(not shown) is formed therein. Next, a first poly silicon film 5 is formed on the upper surfaces of the buffer oxide film 2 and the trench 3, and an impurity is implanted into the first polysilicon film 5. At this time, since the impurity is implanted into all the upper surface of the semiconductor substrate 1, the impurity may be implanted into the active region A of the semiconductor substrate 1. A capacitor oxide film 6 is formed on an upper surface of the first polysilicon film 5, and a first photoresist film 7 is formed on an upper surface of the capacitor oxide film 6, and then patterned for thereby remaining the first photoresist film 7 on the field region F.
Next, as shown in FIG. 1B, the capacitor oxide film 6 and the first polysilicon film 5 are patterned by a photolithography using the patterned first photoresist film 7 as a mask, and the buffer oxide film 2 and the oxide 4 neighboring the patterned first polysilicon film 5 is exposed. The buffer oxide film 2 may be damaged during the photolithography process. The above-described damage is an important problem in the conventional art which will be explained in detail later.
Ions are implanted into the semiconductor substrate for adjusting the threshold voltage of the transistor which will be formed in the next process. Thereafter, the first photoresist film 7 is removed, and the buffer oxide film 2 is removed using a fluoric acid, and as shown in FIG. 1C, a high quality gate oxide film 8 is formed on the region in which the buffer oxide film 2 is removed. However, the oxide 4 filled in the trench 3 which is exposed when exposing the buffer oxide 2 may be damaged. The above-described damage may degrade an isolation characteristic of the semiconductor device. Next, a second polysilicon film 9 is formed on the upper portions of the gate oxide film 8, the exposed oxide 4 and the patterned capacitor oxide film 6. Thereafter, a second photoresist film 10 is formed on a certain region of the second polysilicon film 9 for forming a gate and a gate poly.
Next, as shown in FIG. 1d, the second polysilicon film 9 is patterned by a photolithography using the second photoresist film 10 as a mask, and a gate of the transistor and a gate poly of capacitor are formed on the active region A and the field region F. At this time, a height difference xe2x80x9cdxe2x80x9d is formed between the second photoresist film 10 in the active region A and the second photoresist film 10 in the field region F. Therefore, it is difficult to accurately determine the dimensions of the widths of a gate and a gate poly during the photolithography which is implemented using the second photoresist film. In addition, since the second polysilicon film 9 remains non-etched on the lateral wall of the first polysilicon film 5. Next, the second photoresist film 10 is removed, and a known LDD(Lightly Doped drain) transistor fabrication process is performed, and then as shown in FIG. 1E, the method for fabricating a mixed signal semiconductor device formed of a transistor and a capacitor is completed.
In the above-described method for fabricating the mixed signal semiconductor device, the impurity may be implanted into the active region A of the semiconductor substrate 1 when implanting ions into the first polysilicon film 5, so that the threshold voltage of the transistor formed in the active region A is varied. In order to overcome the above-described problems, the impurity is prevented from being implanted into the active region A of the semiconductor substrate 1 by increasing the thickness of the buffer oxide film 2. In this case, there is a limit for adjusting the thickness of the oxide film, and when the thickness of the buffer oxide film 2 is increased, it takes longer time to remove the buffer oxide film 2, so that a fabrication efficiency is decreased.
In addition, the buffer oxide film 2 is removed before the gate oxide film 8 is formed. At this time, the oxide 4 filled in the trench 3 formed at an exposed portion may be damaged. In particular, the oxide 4 formed at a boundary portion between the active region A and the trench 3 may be significantly damaged. The above-described damage may degrade the isolation characteristic of the semiconductor device.
In addition, a height difference xe2x80x9cdxe2x80x9d is formed between the second photoresist film 10 of the active region A and the second photoresist film 10 of the field region F during the photolithography which is implemented using the second photoresist film 10. Therefore, it is difficult to accurately determined the dimensions of the widths of the transistor and the capacitor.
The characteristic of the capacitor may be degraded due to the second polysilicon film 9 which is formed at the lateral wall of the first polysilicon film 5, and the failed product may be increased, and the fabrication yield may be increased.
Accordingly, it is an object of the present invention to provide a method for fabricating a mixed signal semiconductor device which is capable of implementing a simpler fabrication process and decreasing a failed product.
To achieve the above objects, there is provided a method for fabricating a mixed signal semiconductor device which comprises the steps of a step for dividing a semiconductor substrate into an active region and a field region, step for forming a gate oxide film on an upper surface of the semiconductor substrate, a step for forming a first polysilicon film on an upper surface of the gate oxide film, a step for forming a silicon nitride film on an upper surface of the gate oxide film, a step for patterning the silicon nitride film and exposing a first polysilicon corresponding to the upper portion of the field region, a step for implanting an impurity ion into the first polysilicon film, a step for forming a capacitor oxide film on an upper and lateral surface of the patterned silicon nitride film and on an upper surface of the exposed first polysilicon film, a step for forming a second polysilicon film on an upper surface of the capacitor oxide film, a step for removing the second polysilicon film and the capacitor oxide film formed on the upper surface of the silicon nitride film and planerizing the remaining second polysilicon film, capacitor oxide film and silicon nitride film, a step for patterning the silicon nitride film, first polysilicon film and second polysilicon film, a step for etching the silicon nitride film, and a step for forming a LDD(Lightly Doped Drain) in the active region.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.